1. Field of the Invention
The present invention relates to electrolytic plating and, more particularly, an improved method and electrolytic plating line for plating tin on metallic surfaces, such as steel strip, using insoluble anodes.
2. Description of the Related Art
Tin plating on steel strip using insoluble anodes is known. See, for example, U.S. Pat. No. 4,181,580, the disclosure of which is herein incorporated by reference. This patent teaches a method for electrolytic tinning of steel strip in an electrolytic bath containing tin ions. The bath contains divalent tin ions which are combined with acid and some free acid, that is, acid not combined with tin. When tin plates out, the acid previously combined with tin becomes free. The concentration of tin ions in the bath is controlled by removing electrolyte solution from the bath and contacting the same with particulate tin in a fluidized bed reactor. The net result is to replenish the tin in the bath that has plated out and to take up the free acid that was formerly combined with tin before the tin was plated out. A high content of dissolved oxygen is maintained in the solution by providing additional oxygen to the solution fed into the reactor. Solution replenished with tin is returned to the bath.
However, a significant amount (5-15%) of the dissolved tin in this process becomes tetravalent tin and forms an insoluble oxide sludge. All percentages expressed herein are weight percentages unless otherwise noted. This is a very costly loss of tin. The balance of the tin is dissolved as divalent tin, which is the useful form in the plating process. The loss of tin, caused by the strong oxidizing condition generated by bubbling oxygen, is so serious that many tin plate manufacturers who wish to change to insoluble anodes are remaining with soluble anodes.
It is an object of the present invention to provide a tin plating method which avoids the disadvantage noted above.